s Gas-Phase Radicals in Cigarette Smoke: A Re-evaluation of the Steady-State Model and the Cambridge Filter Pad

Cigarette smoke free radicals have been the subject of many years of investigation. Recently, individual acyl and carbamoyl radicals in fresh whole cigarette smoke were isolated, identified, and quantified by HPLC-mass spectrometry for the first time. These carbon-centered radicals do not conform to the established “steady-state model” for radical formation in gas-phase cigarette smoke. In separate studies, NO2 in fresh whole mainstream smoke has been measured on a puff-to-puff basis by highly precise and rapid tunable infrared diode laser spectroscopy (TILDAS). Only the first puff contained a significant amount of NO2 and its overall yield was substantially lower than previously reported. Nevertheless, when the smoke was passed through a Cambridge filter pad, NO2 appeared in the filtered gas-phase smoke during every puff, suggesting that NO2 forms directly on the pad. For fresh smoke, these results also challenge the applicability of the steady-state mechanism, which postulates that NO2 in the smoke itself is an essential intermediate for gas-phase radical formation. In this review, the historical investigations of cigarette smoke radicals are re-examined and the implications of the new spectroscopic and radical trapping data are discussed. Possible alternative mechanisms for radical formation in cigarette smoke are proposed. When combined with data from previous corroborating reports, these recent results raise serious issues about the use of the Cambridge pad, which is conventionally used to separate gas smoke constituents from whole smoke, but may introduce more artifacts of measurement for certain reactive smoke constituents than previously appreciated.